A photovoltaic device or solar cell is a device that is capable of converting sunlight to electrical energy by the photovoltaic effect. A solar cell, such as a multijunction solar cell, can have one or more component photovoltaic cells, also called subcells. These component photovoltaic cells, or subcells, may be connected in series to form a multijunction solar cell, but may also be connected in other electrical configurations, such as in parallel, or in a combination of series and parallel connections.
The interest in solar cells has been increasing due to concerns regarding pollution, energy security, and limited available resources. This interest has been for both terrestrial and space applications. In space applications, solar cells have been in use for more than 40 years and the development of higher efficiency solar cells enables increased payload capabilities. In terrestrial applications, higher solar cell efficiency for conversion of sunlight to electricity results in a smaller collecting area required for a given electrical power output, and therefore lower cost per watt, and greater cost effectiveness for a terrestrial photovoltaic system.
The cost per watt of electrical power generation capacity for photovoltaic systems inhibits their widespread use in terrestrial applications. The conversion efficiency of sunlight to electricity is typically of crucial importance for terrestrial PV systems, since increased efficiency results in a reduction of all area-related electricity generation system components (such as cell area, module or collector area, support structures, and land area) for a required power output of the system. For example, in concentrator solar cell systems which concentrate sunlight from around 2 to around 2000 times onto the solar cell, an increase in efficiency typically results in a reduction of an area comprising expensive concentrating optics. Improvements in solar cell efficiency are extremely leveraging at the system level, and the dollar per watt ($/watt) ratio is a typical figure-of-merit applied at the system level. For satellites, solar panels represent <10% of the entire system cost so that a relative improvement in solar cell efficiency of 3% over an existing technology generation results in leveraged cost savings. The same is true of terrestrial concentrator solar power systems where the cost of the solar receiver is a fraction of the overall system cost.
To increase the electrical power output of such cells, multiple subcells or layers having different energy bandgaps have been stacked so that each subcell or layer can absorb a different part of the wide energy distribution in the sunlight. This arrangement, called a multijunction (MJ) solar cell, is advantageous, since each photon absorbed in a subcell corresponds to one unit of charge that is collected at the subcell operating voltage, which increases as the bandgap of the semiconductor material of the subcell increases. Since the output power is the product of voltage and current, an ideally efficient solar cell would have a large number of subcells, each absorbing only photons of energy negligibly greater than its bandgap.
In multijunction solar cells it is often desirable to modify the bandgaps of the semiconductor layers that form the subcells within the multijunction cell, and thereby modify the subcell voltages and wavelength ranges over which the subcells respond to incident light, for instance, to space and terrestrial solar spectra. The specific bandgaps and thicknesses of layers that form the subcells within a multijunction cell determine the subcell voltages, the current densities of each subcell, whether the subcell current densities can be matched to one another as is desired in a series-interconnected multijunction cell, and how the broad solar spectrum is divided into narrower wavelength ranges by the combination of subcell bandgaps to achieve higher sunlight-to-electricity conversion. A crucial technological challenge in the design of multijunction solar cells is how to achieve the optimum or near-optimum combination of subcell layer bandgaps, and how to achieve the desired wavelength ranges of subcell response—the wavelength ranges in which the subcells have photogenerated current that can be collected usefully—in order to maximize the multijunction solar cell efficiency. Often the semiconductors that are readily useable—e.g., semiconductors that are lattice-matched to relatively common, inexpensive substrates; that can be grown with favorable minority-carrier properties such as lifetime and mobility; or that do not cause unwanted doping or impurities in other parts of the cell—do not have the bandgaps that result in the most favorable combination of multijunction subcell bandgaps for conversion of the solar spectrum.
In optimum subcell bandgap combinations for multijunction cells under typical space (AM0) and terrestrial (AM1.5 Direct, or AM1.5D) solar spectra, the desired bandgap of the upper subcell—also referred to as the top cell, or cell 1 (C1), also called subcell 1—is often greater than the bandgap of GaInP at the same lattice constant of cell 2 (C2) upon which the top cell is grown. Therefore, it is desirable to use AlGaInP to raise the bandgap of cell 1. However, Al-containing semiconductors often have diminished minority-carrier properties in practice, such as lifetime, mobility, and diffusion length, compared to their Al-free counterparts, resulting in reduced current collection from Al-containing solar cell layers. This is particularly evident when the top cell emitter is formed from n-type AlGaInP. Thus there is a need for solar cell layers with improved minority-carrier properties and current collection that can be used in a top cell for which the main absorber layer is high-bandgap AlGaInP.
Additionally, in multijunction solar cells some photogeneration layers—defined here as layers in which charge carriers are photogenerated and can be collected, including in a useful manner—have low absorptance due to other design constraints in the solar cell. It is desirable to increase photogeneration in these weakly absorbing structures, and thereby increase current density of the subcell and multijunction cell.
Past approaches to increasing photogenerated current density include increasing the thickness of current generating regions for which there is insufficient light absorption above the bandgap. However, in many cases, absorption of light by the solar cell with photon energy above the solar cell bandgap is nearly complete, so increasing the thickness has little effect on the current, or can cause the current to decrease due to poorer collection of photogenerated charge carriers from thicker solar cell layers. Another approach has been to lower the bandgap of the semiconductors used to form the current generating regions of a solar cell. However, this approach also lowers the solar cell voltage. In addition, lowering the bandgap by changing the semiconductor composition often changes the crystal lattice constant, creating a greater lattice mismatch with other layers in the solar cell, which can lead to a higher density of harmful dislocations in the lattice-mismatched subcell.
There exists a need for solar cells and other optoelectronic devices having 1) semiconductor layers that photogenerate a greater quantity of charge carriers; 2) semiconductor layers that facilitate collection of photogenerated minority charge carriers in the solar cell to form useful current density; 3) a more nearly optimum combination of subcell bandgaps and subcell wavelength response ranges in multijunction devices for better energy conversion efficiency; and 4) device structures that increase the photogeneration in and usefully collected current from weakly absorbing or incompletely absorbing photogeneration layers in the solar cell.